1. Field of the Invention
The present invention relates to a DC stabilized power supply, and more particularly to a switching signal generator for use in a switching power supply employing a xcex94 xcexa3-modulator, in which a switching signal for a power switch element is fed back from a gate driver circuit output to the modulator, and to a DC-DC converter utilizing xcex94 xcexa3-modulation in which the gain of an integrator inside a xcex94 xcexa3-modulator can be adjusted in accordance with output of the DC-DC converter.
2. Description of the Related Art
Description will first be made in connection with the switching signal generator for the switching power supply.
A conventional PWM (pulse-width modulation) switching signal generator modulates an input signal by varying its pulse width. In other words, as shown in the block diagram of a PWM switching signal generator in FIG. 5, a PWM oscillator 18 generates a gate signal from an input signal 1, which gate signal is amplified by a gate driver circuit 6 to drive a power switch element 7. In the PWM system, the distortion produced at the gate driver circuit 6 has been unable to be corrected.
A block diagram of a conventional PWM switching signal generator applied to a step-down chopper is shown in FIG. 6. The voltage applied to a load and a reference voltage 17 are compared, and its result is inputted into a PWM oscillator 18 so that a power switch element 7 is controlled by the output of the PWM oscillator 18. In this generator, however, because there is provided no direct feedback path from the gate driver circuit 6, the distortion generated at the gate driver circuit 6 has been impossible to correct.
Another conventional switching signal generator has been known in which, as shown in FIG. 7, a xcex94 xcexa3-modulator 5 is used to output a gate drive signal to and drive a power switch element 7. In the xcex94 xcexa3-modulator 5, an input signal is integrated by an integrator 3 instead of using the PWM oscillator and quantized to provide a one-bit output signal. With this generator, however, because the feedback is performed upstream of a gate driver, the distortion produced at the gate driver circuit 6 may not be corrected.
A block diagram of a conventional xcex94 xcexa3 switching signal generator applied to a step-down chopper is shown in FIG. 8. The voltage applied to a load and a reference voltage 17 are compared, and its result is inputted into a xcex94 xcexa3-modulator 5 so that a power switch element 7 is controlled by the output of the xcex94 xcexa3-modulator 5. In this generator, however, because there is provided no direct feedback path from the gate driver circuit 6, the distortion generated at the gate driver circuit 6 has been impossible to correct.
As a result, because distortion has not been removed from the gate driver circuit 6 which directly drives the power switch element 7, the linearity of the xcex94 xcexa3-modulator has been impaired.
Due to the distortion produced at the gate driver circuit both in the conventional PWM system and xcex94 xcexa3 system, there is caused an error between, in the PWM system, the output signal of the PWM oscillator and the gate driver output signal and, in the xcex94 xcexa3 system, between the output signal of the xcex94 xcexa3-modulator and the gate driver output signal. Thus, when these conventional switching signal generators are used for controlling a switching power supply, especially when operated at a high frequency, there will be obtained only a small phase margin, resulting in unstable control.
Furthermore, the addition of a phase correction circuit, which is necessary to prevent oscillation, increases the number of parts and thus the cost. In addition, with these conventional generators, in order for an optimum circuit to be designed, an experiment with an actual circuit used has been conducted in many cases, resulting in stable circuit design being made difficult and increasing in development time.
Another method has been known in which the linearity is improved by inputting an analog signal and feeding back the output signal of a power switch element, as in a switching amplifier utilizing xcex94 xcexa3 modulation proposed in Japanese Patent Application Laid-Open Specification No. 2000-307359.
FIG. 9 is a block diagram of a conventional xcex94 xcexa3-modulation system in which the output signal of a power switch element is fed back. This conventional method, however, has the following drawbacks when used for controlling a switching power supply.
If the output current of a switching power supply is small, there will occur a discontinuity region in which the current flowing through an inductance located in a power supply circuit becomes discontinuous. If in this discontinuity region the current flowing through the inductance becomes zero, an oscillation is caused by a capacity component and an inductance component inside the power supply circuit.
As shown in FIG. 9, with the method of feeding back the output signal of a power switch element to a xcex94 xcexa3-modulator, the noise caused by this oscillation is also fed back, thereby considerably increasing the number of switchings of the xcex94 xcexa3-modulation output. Consequently, an increase is made in the switching loss, and a reduction is made in the power supply efficiency.
In other words, with a conventional switching signal generator, because the distortion produced at the gate driver is difficult to remove, an impaired linearity of direct-current transmission characteristics results, which in turn results in a small phase margin, thereby readily giving rise to oscillation.
With the conventional xcex94 xcexa3-modulation-utilizing method in which the feedback path extends from downstream of the power switch element to improve the linearity, a reduction is made in the power supply efficiency due to the noise caused by the current discontinuity.
Description will now be made in connection with the DC-DC converter utilizing the xcex94 xcexa3-modulation.
xcex94 xcexa3-modulation is a modulation system in which an input signal is integrated, the integrated value is compared with a reference voltage to perform quantization, and its output is fed back to a modulator input. Shown in FIG. 13 is a block diagram of a primary xcex94 xcexa3-modulator. By using this modulation system, switching of a switching element can be performed to make a DC-DC converter.
With a conventionally widely-used DC-DC converter employing a pulse width modulation system (PWM), the switching frequency is constant, whereas with a DC-DC converter utilizing xcex94 xcexa3-modulation, the switching frequency varies responsive to a converter output. Thus, the latter has an advantage that a reduction may be made in the switching loss under light load, and has attracted attention.
Shown in FIG. 14 is one example of a conventional step-down chopper DC-DC converter utilizing xcex94 xcexa3-modulation in which, comparison is made between a converter output voltage and a reference voltage to xcex94 xcexa3-modulate an error-amplified signal voltage of the compared voltages as an input voltage of the xcex94 xcexa3-modulator, switching of a switching element is made by the output signal of the modulator, and the switching output is inputted into a smoothing circuit to obtain a constant voltage output.
The xcex94 xcexa3-modulator has at least one integrator, and shown in FIG. 15 is one example of an integrator employing an operational amplifier. The gain that represents the gradient of variation of the output voltage relative to the input voltage of the integrator, is determined, if the frequency component of the signal inputted into the integrator is within the operational amplifier band, only by the value of resistance and the capacitance value irrespective of the gain of the operational amplifier, and is proportional to the inverse of the product of the resistance value and the capacitance value.
Where xcex94 xcexa3-modulation is employed in a DC-DC converter, care must be taken so as not to saturate the output of an integrator located inside the xcex94 xcexa3-modulator, otherwise an accurate modulation will not be achieved, with the result that constant-voltage control of the DC-DC converter becomes unstable, making it impossible to maintain constant the output voltage of the DC-DC converter.
Due to the above, the gain of an integrator has conventionally been determined so as not to saturate the integrator output. In other words, the amplitude of an integrator generally becomes great when the output current of a DC-DC converter is small, and hence, in order for the integrator not to become saturated at that time, it has been necessary to set the gain low.
With the above conventional method, the amplitude of the integrator becomes small when the output current of the DC-DC converter is great. Because the gain is set low so that the integrator output will not become saturated when the output current is small, there has arisen a problem that the amplitude of the integrator output is of small value less than the order of mV.
The quantizer of a xcex94 xcexa3-modulator requires a comparator for comparing the integrator output with a reference voltage. The difference between the two voltages that are compared at the comparator is called an overdrive. If the overdrive is large, the variation of the output voltage of the comparator becomes rapid, making a high-speed operation possible, whereas in contrast, if the overdrive is small in amount, a high-speed operation cannot be realized.
The minimum overdrive amount generally required for a comparator to operate at a high speed is from several tens of mV to 100 mV In order to increase the overdrive amount and enable the comparator to operate at a high speed, it has been necessary to increase the gain of the integrator.
In the conventional method, however, because, for the purpose of preventing saturation of the integrator output, the gain of the integrator is fixed low in conformity with the time when the amplitude of the output voltage of the integrator is greatest, the comparator may not be given a sufficient overdrive when the output current of the DC-DC converter is great, and thus the amplitude of the output voltage of the integrator is small, making it impossible for the comparator to operate at a high speed.
The low-speed operation of the comparator causes the feedback of the DC-DC converter to be at a low speed, and there has been a problem that, if switching, especially high-frequency switching of the DC-DC converter is made, it gives rise to oscillation.
In other words, with the conventional method, there has been a trade-off in that, if the gain of the integrator is fixed low so as not to saturate the integrator output, the DC-DC converter undergoes oscillation, and if the gain of the integrator is fixed high so as not to cause oscillation of the DC-DC converter, the integrator output becomes saturated and the output voltage becomes unstable.
Thus, with the conventional method, due to this trade-off, the gain of the integrator must be made low to such a degree as not to saturate the integrator output, and must be made high to such a degree as not to cause oscillation of the converter, resulting in designing difficulty and, in addition, in difficulty in realizing a fast-operating converter without causing saturation of the integrator.
The present invention has been made to overcome the above drawbacks, and accordingly, it is an object of the present invention to provide a switching signal generator for use in a switching power supply employing a xcex94 xcexa3 modulator, which is of high efficiency, provides a large phase margin, and is stably controllable during switching at a high frequency.
It is another object of the present invention to provide a DC-DC converter employing xcex94 xcexa3-modulator, which is stably controllable during switching at a high frequency and produces a stable output voltage without undergoing oscillation even at a high sampling frequency.
In order to attain the above objects, the present invention according to one aspect thereof is characterized in that, in a switching power supply in which an analog input signal or a multi-bit digital signal is inputted into a xcex94 xcexa3-modulator, and the modulated signal is amplified at a gate driver circuit to perform switching of a power switch element with the thus amplified signal, the gate driver output is fed back to the xcex94 xcexa3-modulator.
The xcex94 xcexa3-modulator is constituted by at least one adder, one integrator, and one quantizer. The feedback path extends from the output of the gate driver circuit connected to a quantizer output to at least one adder input. The at least one integrator is connected to an adder output and has at least one output connected to the quantizer.
By providing two or more integrators in series connected to the adder output, or by providing two or more integrators in parallel connected to the adder output, the accuracy of sampling will be enhanced by the former, and a parallel arrangement of outputs will be obtained, making it possible to provide a precision or multi-output switching power supply for a small-sized integrated circuit.
Where a continuous-time signal such as an analog signal is inputted into the xcex94 xcexa3-modulator, an analog adder and an analog integrator are used as the adder at the input of the modulator and the integrator connected to the adder output, respectively, and where a discrete-time signal such as a multi-bit digital signal is inputted into the modulator, a digital adder and a digital integrator may be used.
The quantizer is a quantizer that performs sampling on the discrete-time signal and is connected to the input of the gate driver circuit that receives an output signal of the quantizer and supplies the current/voltage sufficient to drive the power switch element. Such a gate driver circuit is needed in case the quantizer output is insufficient to drive the power switch element and is needed for the fed-back signal to fully show its effect.
The feedback path may include an attenuator that adapts a pulse signal of great amplitude of the gate driver circuit to the level of the input signal of the xcex94 xcexa3-modulator.
Because the feedback path is provided that extends from the output of the gate driver connected to a quantizer output to a modulator input, a reduction is made in the distortion produced at the gate driver. In other words, as compared with the conventional PWM system and the conventional xcex94 xcexa3-modulator in which feedback is made from immediate downstream of the quantizer, the distortion produced at the gate driver may be directly fed back.
Because direct feedback is made from the output of the gate driver circuit connected to the quantizer output to the modulator input, a satisfactory linearity of input and output characteristics may be obtained, leading to a large phase margin and unlikeliness of oscillation.
By direct feedback from the gate driver output, the distortion produced at the gate driver circuit may be directly fed back, and as compared with the case where the feedback is made from downstream of the power switch element, less noise is produced, and when the switching signal generator is applied in a switching power supply, a reduction can be made in the switching loss.
It is apparent that by making the analog input signal direct-current voltage, the output is amplified into a direct-current power, making it possible for the switching power supply to function as direct-current switching power supply. It is also possible to make up a xcex94 xcexa3-modulation type chopper power supply by adding a rectifier smoothing circuit downstream of the power switch element.
The present invention according to another aspect thereof is characterized in that, in a DC-DC converter in which an analog input signal or a multi-bit digital signal is inputted into a xcex94 xcexa3-modulator to perform switching of a switching element with the thus modulated signal, the gain of an integrator inside the xcex94 xcexa3-modulator is adjusted in accordance with conditions of the converter.
The xcex94 xcexa3-modulator is constituted by at least one adder, one integrator and one quantizer, a feedback path is provided to extend from the xcex94 xcexa3-modulator output to at least one adder input, and the at least one integrator is connected to an adder output and has at least one output connected to the quantizer.
The quantizer performs sampling on a discrete-time signal and is connected to the input of a gate driver circuit. The gate driver circuit receives an output signal of the quantizer and supplies the current/voltage sufficient to drive the power switch element.
The at least one integrator is provided with a gain-adjusting circuit. In other words, a current flowing inside the DC-DC converter, voltage inside the converter, or converter output voltage is detected, and by controlling either one or both of the resistance value and the capacitance value as indicated, for example, in FIG. 15 by the detected signal, the gain of the integrator may be adjusted.
Furthermore, it is possible to directly detect the output voltage of the integrator and, in accordance with the detected signal, adjust the gain of the integrator within the range in which the gain of the integrator is so low as not to saturate the amplitude of the output voltage of the integrator and is so high as to allow high-speed operation of the comparator.
Because the gain of the integrator may be adjusted within the range in which the output voltage amplitude is not saturated while the comparator can operate rapidly, by enhancing the gain at the time when the integrator amplitude is small, the overdrive given to the comparator may be increased, and thus a high-speed feedback and a stable constant voltage control of the converter may be realized.
In contrast, by reducing the gain at the time when the integrator amplitude is great, saturation of the integrator output may be prevented, leading to a stable constant voltage control.
Furthermore, if a variation is made in the gain of the integrator, such a variation is corrected by the feedback from the xcex94 xcexa3-modulator output, and thus the input and output linearity of the modulator will not be impaired, and the operation of the modulator will be maintained stable.
In other words, according to the another aspect of the present invention, a stable DC-DC converter is provided in which, relative to the operating condition of the DC-DC converter, the gain of the integrator may be adjusted within the range in which the amplitude of the output voltage is not saturated and the comparator can operate at a high speed, and which does not undergo oscillation and suffers from few fluctuations in the output voltage.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.